Different instructions cause different pathways to be chosen. For example, STD causes the value in A to be copied into MBR and then a memory write operation is activated. A doesn't get changed in the process, but some word of memory gets overwritten with A's value.

The control unit uses the current instruction to activate a sequence of pathways to get the proper job done. If the ADD wire is high, the sequence shown above is always done. If the STD wire is high instead, a different sequence is done. The first couple of steps, up through the instruction fetch stage and incrementing of the PC, are always done by every instruction, even HLT and NOP. From that point, differences occur depending upon which instruction is being performed.